1. Field of the Invention
This invention relates to a semiconductor laser apparatus with an improved performance that is used for optical measurements.
2. Description of the Prior Art
When a semiconductor laser is used for optical measurements, a non-linear relationship between the injection-driving current and the optical output power of the semiconductor laser and/or a variation in the oscillation wavelength of the semiconductor laser drastically lower the precision of the said measurements. For instance, when an optical measurement is carried out utilizing the interference between the laser light from a semiconductor laser and the reflected light from an object to be measured, the interference pattern varies with a variation in the oscillation wavelength, which makes such a measurement difficult.
FIG. 5 shows the structure of a conventional semiconductor laser apparatus, which comprises a housing 10, a semiconductor laser device 100 disposed within the housing 10, and a wiring 6 connecting the semiconductor laser device 100 to the external power source. The housing 10 is constituted by a base 2, a heat sink 3, side walls 4 and a glass window 5. The semiconductor laser device 100 is mounted on the heat sink 3, which is supported by the base 2. The semiconductor laser device 100 is connected to a lead pin 7 by the wiring 6 made of Au or Al. The lead pin 7 extends to the outside of the housing 10 through the base 2. When driving current is injected into the semiconductor laser device 100 through the lead pin 7 and the wiring 6, laser light is emitted from the semiconductor laser device 100 and radiated outside of the housing 10 through the glass window 5. The glass 50 constituting the glass window 5 is disposed in the direction that is vertical to the proceeding direction of the laser light so as to prevent aberration by the glass 50, so that a part of the laser light with which the window glass 50 is irradiated is reflected by the window glass 50 and returns to the semiconductor laser device 100. Accordingly, due to the etalon characteristics of the semiconductor laser device 100 that depend upon the distance between the semiconductor laser device 100 and the window glass 50, the phenomenon arises that the facet of the semiconductor laser device 100 exhibits a higher effective reflectivity for laser light with a certain wavelength and a lower effective reflectivity for laser light with another wavelength. The said characteristics periodically arise. The space .DELTA..lambda.e between the said wavelengths periodically emerges and is represented by the equation (1): EQU .DELTA..lambda.e=.lambda..sub.0.sup.2 /2d (1)
wherein .lambda..sub.0 is the oscillation wavelength of laser light and d is the distance between the semiconductor laser device 100 and the window glass 50.
On the other hand, the space .DELTA..lambda. between the adjacent longitudinal modes of the semiconductor laser device 100 is represented by the equation (2), as is well known: EQU .DELTA..lambda.=+.sub.0.sup.2 /2nl (2)
wherein n is the effective refractive index of a resonator of the semiconductor laser device and l is the internal-cavity length.
FIG. 3 shows the relationship between the reflection gain represented by the equation (1) and the longitudinal mode of laser light in the case that .DELTA..lambda.e=1.5.DELTA..lambda.. Provided that, as shown in line (b) of FIG. 3, a longitudinal mode D of laser light agrees with a high portion of the reflection gain indicated by reference mark (a) that is positioned near the peak of gain of the semiconductor laser device 100, when the longitudinal mode moves relatively toward the longer wavelength region due to changes in currents and/or temperatures, then the longitudinal mode E that is adjacent to the longitudinal mode D agrees with the high portion of the reflection gain as shown in line (c) of FIG. 3. Moreover, the longitudinal mode C, likewise, agrees with the high portion of the reflection gain as shown in line (d) of FIG. 3. In this way, the oscillation mode gives rise to mode hopping (i.e., changes in the longitudinal oscillation mode) such as that from mode D to mode C through mode E (i.e., D.fwdarw.E.fwdarw.C). Even when the longitudinal mode D is maintained without mode hopping, optical output power of the semiconductor laser device 100 is not linearly proportional to currents and/or the ratio of the oscillation mode to the non-oscillation mode is markedly reduced.
The above-mentioned problems also arise in lasers such as internal reflector interferometric lasers, composite cavity laser devices, external resonator type lasers, etc., that have been proposed so as to stabilize the longitudinal oscillation modes. Distributed feedback (DFB) lasers also give rise to the problems that mode hopping between the two DFB modes sandwiching a stop band therebetween arises and that the ratio of the oscillation mode to the non-oscillation mode with a space that can be represented by the equation (2) varies. In order to eliminate the above-mentioned problems, a process by which the window glass is coated with a film for the prevention of a laser light reflection has been developed. However, such a film must be made with multi-layered dielectric films, etc., so as to create a low reflectivity, which necessitates highly accurate and complicated production processes.